Method of producing deep drawing steel

ABSTRACT

A ferrous material capable of extremely deep drawing is obtained from rimmed carbon steel containing not more than 0.10 percent C by hot rolling at a finishing temperature of at least 800* C., rapid quenching to less than 500* C., slow further cooling to permit cementite precipitation, cold rolling, and recrystallizing by annealing at 650* to 800* C., the annealing temperature being reached at a heating rate of less than 100* C./hr.

United States Patent Shiraiwa et al.

[54] METHOD OF PRODUCING DEEP DRAWING STEEL [72] Inventors: Toshio Shiraiwa, Nara-Ken; Fukunaga Terasaki, Osaka, both of Japan [73] Assignee: Sumitomo Metal Industries, Ltd., Osaka,

Japan [22] Filed: Nov. 17, 1970 [21] Appl. N0.: 90,450

Related US. Application Data [63] Continuation-in-part of Ser. No. 18,350, Mar. 10,

1970, abandoned.

[30] Foreign Application Priority Data Mar. 13, 1969 Japan ..44/19409 U.S.Cl ..l48/l2,148/12.1,148/123 1 May 16, 1972 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-W. W. Stallard Attorney-Kelman and Berman [57] ABSTRACT A ferrous material capable of extremely deep drawing is obtained from rimmed carbon steel containing not more than 0.10 percent C by hot rolling at a finishing temperature of at least 800 C., rapid quenching to less than 500 C., slow further cooling to permit cementite precipitation, cold rolling, and recrystallizing by annealing at 650 to 800 C the annealing temperature being reached at a heating rate of less than 100 C./hr.

5 Claims, No Drawings METHOD OF PRODUCING DEEP DRAWING STEEL This application is a continuation-in-part of our co-pending application Ser. No. 18,350, filed on Mar. 10, 1970, and now abandoned.

This invention relates to the production of low-carbon rimmed steel capable of being deep-drawn.

It is common practice to convert an ingot of rimmed steel to a slab on a blooming mill, reheat the slab, and to pass the reheated slab through a continuous hot rolling mill to produce strip which is then coiled while being water cooled. The hot rolled strip is pickled to remove scale, cold-rolled to further reduce its thickness, and the cold-rolled material annealed to soften it by final recrystallization.

The depth of draw to which such a product can be subjected depends largely on the kind, distribution, and quantity of precipitated non-metallic phases such as AlN or Fe C, and it has been proposed to subject the hot-rolled stock to solution heat treatment and further processing to improve its Conical Cup Value (C.C.V.) which is a standard measure of drawability. The known methods, as far as they have been effective in producing deep drawing steel of exceptional drawing properties, have not been suitable for industrial application.

The primary object of this invention is the provision of a method of producing a cold-rolled, low carbon steel which is capable of industrial application on a large scale, and which produces steel having superior drawing properties.

It has been found that the steel must be converted to the ultimate deep drawing material by a multiple step treatment at temperatures of which some are critical, and must be brought from one temperature to another at a rate which is critical in most instances, as will presently become apparent.

According to this invention, a rimmed carbon steel containing not more than 0.10 percent carbon (and not less than 0.02 percent by definition) is hot-rolled into a strip of predetermined thickness at a finishing temperature of not less than 800 C., and not normally higher than 900 C. to avoid grain growth. The hot-rolled strip is quenched from the finishing temperature to not more than 500 C., but not normally below 250 C. at a rate sufficient to prevent cementite precipitation at the grain boundaries, and further cooled thereafter at a slow rate, normally to ambient temperature. Thereafter, the strip is subjected to cold work by rolling, as may be needed for further reduction of its thickness, and then to a slow, final annealing treatment for recrystallization in which the crystal planes (ll 1) parallel to the strip surface are increased, and cementite is precipitated in finely dispersed form.

Finish rolling at 800 to 900 C. causes cementite to go into solid solution, and it is dispersed in finely distributed form by quenching below 500 C., and preferably to 450 to 300 C. If the heating rate during ultimate recrystallizing is lower than 100 C. per hour, and is preferably held to 40 to 80 C. per hour, a steel capable of extremely deep drawing is obtained.

Further cooling after quenching is preferably achieved by coiling the quenched strip and exposing the coil to ambient air. Pickling and descaling normally precedes cold rolling, as is conventional if hot rolling was performed in air, as is normally the case. Cold rolling should reduce the cross section of the steel by 50 percent to 90 percent to achieve the desired softening and recrystallization at the preferred annealing temperature of 650 to 750 C.

The exact nature of this invention as well as additional objects and many of the attendant advantages will readily be appreciated from the following illustrative examples.

EXAMPLE 1 Samples of low-carbon rimmed steels having the compositions A to E listed in Table l, and essentially consisting of iron and less than 0.01% Si, as far as not specifically stated otherwise, were subjected to hot rolling to a thickness of 3.2 millimeters, unless stated otherwise, on laboratory equipment known to duplicate the conditions on a continuous, hot, strip finishing mill. All samples were later cold-rolled to a thickness of 0.8 mm and annealed in a decarburizing atmosphere of TABLE 1 Sample No. C Mn P S A 0.030 0.37 0.015 0.015 B 0.044 0.27 0.009 0.014 C 0.043 0.27 0.009 0.037 D 0.074 0.34 0.008 0.021 E 0.048 0.31 0.012 0.015

Table 2 shows the influence of the quenching temperature and of the heating rate during the ultimate annealing on the C.C.V. of Samples A and B which were water quenched after hot rolling at a finishing temperature of 850 C. coiled, and thereafter slowly cooled to room temperature in a furnace from quenching temperatures of 600 C. or 500 C. to duplicate with the small sample coils a cooling rate obtained by air cooling a coil of commercial size. The samples were then descaled by pickling in the usual manner, cold-rolled to 0.8 mm as indicated above, and annealed, being heated to the ultimate temperature at rates varying between 5 and 160 C. per hour in a neutral (N) or a decarburizing (D) atmosphere.

TABLE 2 Conical Cup Value, mm

Quenching Temp. C. 600 500 Sample A B A B 5C./hr in N atm. 36.9 36.5 10C. 37.4 37.2 20C. 37.3 37.0 35.0 40C. 37.4 37.3 35.9 C. 37.5 37.3 37.2 36.6 C. 37.4 37.3 37.2 36.5 10C./hr in D atm. 36.7 36.9 35.3 40C. 36.6 36.8 160C. 36.8 36.9 35.6 36.1

The table clearly shows the superiority of slow cooling from a quenching temperature of 500 C. as compared to quenching to 600 C. under otherwise identical conditions. It also shows that the benefits of slow heating to the ultimate annealing temperature after cold working are available only after quenching to 500 C., and that the samples quenched to 600 C. showed closely similar C.C.V. regardless of the reheating rate to the annealing temperature. The advantages of quenching to 500 C. are small at reheating rates of 80 C. or higher. A decarburizing atmosphere used during annealing does not differ greatly in its effect on the C.C.V. from a nondecarburizing atmosphere, but is somewhat superior at rates of temperature increase up to 40 C.

Table 3 illustrates the relationship of the temperature during finishing rolling and the C.C.V. on samples which were annealed after cold working in a neutral atmosphere, at various reheating rates.

The importance of a finishing temperature of 800 to 900 C. on the rolling mill is evident from Table 3. The grain growth occurring above 900 C. makes it further inadvisable to finish roll at temperatures significantly above 900 C.

Table 4 indicates the advisability of immediately further cooling, though at a slow rate, from the chosen quenching temperature. Samples of composition B were hot rolled at a finishing temperature of 850 C., quenched with water to 400 450 C., coiled, and furnace cooled slowly from starting temperatures between 300 and 500 C. with the exception of the first test in which a temperature of 500 C. was maintained for 1 hour before cooling began. The quenched and further cooled samples were pickled and cold worked, as described above, and reheated to 710 C. at the rates listed.

( l Held at 500C one hour before cooling.

The influence of cold rolling on the C.C.V. is hown in Table 5 which also shows the effects of quenching from 850 C. to 600 or 400 C. after hot rolling, and of rates of reheating to annealing temperature at 20, 40, and 80 C. per hour. The final thickness of the cold-rolled stock was 0.8 mm in all cases, but the hot-rolled thickness was varied to provide the indicated reduction in area. All samples were of composition C.

TABLE 5 Qeunchg Redn of Conical Cup Value, mm Temp. "C Area. 20C/hr 40C/hr 80C/hr The effects of a greater reduction in area during cold-rolling on the C.C.V. is barely measurable in the steel samples quenched to 600 C., but significant in those quenched to 400 C., with subsequent slow cooling under conditions simulating the air cooling of a large coil, as described above. Table 5 also confirms the beneficial effects of a slow heating to the annealing temperature when preceded by quenching from the finishing temperature after hot rollingat a temperature of 500 C. or less.

EXAMPLE 2 Under actual mill conditions, a slab of rimmed steel having the composition E in Table 1 was passed through a hot strip mill and emerged at a thickness of 3.2 millimeters and a finishing temperature of 810 to 860 C. it was quenched with water to 350 400 C., coiled while still at the quenching tempera ture, and the coil was slowly cooled by ambient air to room temperature.

The strip was then pickled and descaled, and cold-rolled to 1 a thickness of 0.8 mm with a reduction of cross sectional area of 75 percent. Test pieces taken from the cold-rolled strip were armealed in a neutral (N) atmosphere or decarburizing (D) atmosphere for 16 hours at 710 C., being heated to that temperature at rates varying between 5 and 320 C./hr. For comparison purposes, a control piece was reheated at 650 C. for 30 minutes and slowly cooled in a furnace to simulate the condition of the same material when quenched to about 650 C. The annealing treatment was the same as for the production samples.

As is evident from the comparison tests of example 1, and further confirmed by the test of Example 2 perfon-ned under actual steel mill conditions, hot rolled strip quenched to a temperature of 500 C. or less, and thereafter slowly cooled, cold worked, and raised to the annealing temperature at a rate of less than 100 C./hr has deep drawing properties substantially superior to an otherwise identical steel which was processed under conventional conditions of quenching to 600 C. and/or reheating to the annealing temperature at a rate of 100 C./hr or more. A low reheating rate is not beneficial unless following quenching to a relatively low temperature, and quenching to a low temperature is not in itself effective unless followed by cold working and heating to the annealing temperature at a rate of less than 100 C./hr. There is no lower limit to the effective heating rate, and the quenching temperature may be reduced to as low as 250 C. without loosing all benefits of this invention.

The annealing conditions are not otherwise critical, and normal practice may be followed. Conventional bright annealing or annealing under conditions to induce decarburization or removal of nitrogen, as in a wet hydrogen atmosphere, may be resorted to. Heating rates of less than 20 C./hr, hr, while eminently effective, cannot usually be maintained under mill conditions where equipment must be used efficiently, but heating rates of 40 to C./hr are both practical and effective. The ultimate annealing temperature may be chosen in a conventional manner, and will normally be between 650 and 800 C. No unexpected results have been observed from varying the annealing temperature within this range, and the time required for recrystallization is inversely related to the temperature. The annealing time also may be chosen according to conventional practice, and has no unexpected bearing on the results of this invention.

it should be understood, therefore, that the invention is not limited to the specific conditions chosen in the examples for the purpose of the disclosure, but is to be construed broadly and restricted solely by the scope of the appended claims.

What is claimed is:

l. A method of producing a deep drawing steel which comprises:

a. hot rolling a rimmed carbon steel containing 0.02 to 0.10

percent carbon to a predetermined cross sectional area at a finishing temperature of not less than 800 C., but not substantially higher than 900 C.,

b. quenching the rolled steel from said finishing temperature to a quenching temperature of less than 500 C., but not substantially lower than 250 C. at a rate sufiicient to prevent significant precipitation of cementite at the grain boundaries;

c. slowiy cooling the quenched steel from said quenching temperature at a rate sufficient to permit cementite precipitation;

d. subjecting the cooled steel to cold rolling until said area is reduced;

e. heating the cold-rolled steel to an annealing temperature of 650 to 800 C. at a rate of less than 100 C. per hour;

f. holding the heated steel at said annealing temperature until recrystallized; and

g. cooling the recrystallized steel to ambient temperature.

2. A method as set forth in claim 1, wherein said quenching temperature is 300 to 450 C.

3. A method as set forth in claim 1, wherein said quenched steel, prior to said cooling, is formed into a coil, the coil being exposed to ambient air to effect said slow cooling of the quenched steel, the steel being cooled to a temperature sufiiciently low to permit said cold rolling.

4. A method as set forth in claim 1, wherein said area is reduced by 50 to percent during said cold rolling, the quenched steel being descaled prior to said cold rolling.

5. A method as set forth in claim 1, wherein said annealing temperature is not higher than 750 C. 

2. A method as set forth in claim 1, wherein said quenching temperature is 300* to 450* C.
 3. A method as set forth in claim 1, wherein said quenched steel, prior to said cooling, is formed into a coil, the coil being exposed to ambient air to effect said slow cooling of the quenched steel, the steel being cooled to a temperature sufficiently low to permit said cold rolling.
 4. A method as set forth in claim 1, wherein said area is reduced by 50 to 90 percent during said cold rolling, the quenched steel being descaled prior to said cold rolling.
 5. A method as set forth in claim 1, wherein said annealing temperature is not higher than 750* C. 